Following up An Oblique Approach and In the Heart of Darkness, Flint and Drake continue their primitive warfare series set in a Roman empire that has been changed by the arrival of a time traveler. Known as ""Aide,"" this crystalline entity, when held in the hand of the Roman general Count Belisarius, reveals enough of future technology and history that the Roman army in A.D. 531 is equipped with rockets and portable cannons, and supported by a network of semaphore towers. Ostensibly, Aide's motive is to help the Romans change the course of history by defeating their enemies, the Malwa, based in India, so that, in the distant future, Aide's people will not be enslaved by ""the Great Ones."" The Romans do alter the future through their actions, but the authors neither address the larger implications of time-travel paradoxes, nor reveal any of Aide's deeper motives. Instead, they dwell on the rather straightforward adventures of Belisarius, who leads the Roman army against the Malwa; his wife, Antonina, who spearheads the development of Roman weaponry; and the eight-year-old Roman emperor, Photius. The battle scenes and strategies are as expert as expected in a book with Drake's name on it, but the ending, though superficially winding up the series, has an inconclusive feel, as if there's at least one more episode to come. (July)